Best Audio Format for Audiobooks
Best audio formats for audiobooks — ACX, Audible, iTunes, and personal listening. Bitrate, mono vs. stereo, and delivery specs.
Audiobook audio has specific requirements that differ from music: mono is often preferred, loudness standards are stricter, and file sizes matter more because books are long. Here is the complete format guide for audiobook production and distribution.
ACX Requirements (Amazon/Audible)
ACX is the primary marketplace connecting narrators with publishers and self-publishing authors. Their technical requirements are among the strictest in the industry. Format: MP3, 192 kbps CBR (constant bitrate). Sample rate: 44.1 kHz. Channels: mono. Loudness: -23 to -18 dB RMS. Peak: no higher than -3 dBFS. Noise floor: -60 dB or lower from the RMS level. Files need at least 0.5 seconds of room tone at both the beginning and end.
iTunes Audiobooks
Apple Podcasts and the Books app on iPhone use M4A (AAC) or M4B format. M4B is an MPEG-4 audiobook file that supports bookmarking — the player remembers your position when you close the app. M4B is identical to M4A in audio format but has a different file extension and MIME type that tells players to enable bookmarking. For iTunes Store submission, Apple requires M4B for audiobooks. For personal use, M4A works in the Books app.
Mono vs. Stereo
Most professional audiobooks use mono audio. Narration does not benefit from stereo — the voice comes from one source (the narrator) and stereo adds file size without improving the listening experience. ACX requires mono. For personal audiobook projects, mono at 128 kbps MP3 is extremely space-efficient (roughly 1 MB per minute) while sounding perfectly natural for spoken word.
For Personal Listening
MP3 at 64–128 kbps, mono, 44.1 kHz is the best format for personal audiobook listening. At 64 kbps mono, voice quality is excellent and file size is tiny — a 10-hour audiobook is under 300 MB. M4A at 64 kbps mono is slightly more efficient (better codec) and plays natively on iPhone. OGG Vorbis at 64 kbps is excellent for Android with similarly tiny file sizes.
Production Workflow
Record narration as WAV at 48 kHz, 24-bit, mono. Edit and master in WAV. Export as WAV for submission to mastering engineers. For ACX self-submission: export as MP3 at 192 kbps CBR, 44.1 kHz, mono after mastering. Verify loudness with Youlean Loudness Meter before submitting.